This project is designed to provide a detailed understanding of those factors governing DNA flexibility. Such information is of central importance in helping to resolve the broader issue of how extremely long DNA molecules arrange themselves within such compact structures as virus particles and chromosomes. Recombinant DNA molecules of precisely defined sequence and length will be used in conjunction with electric birefringence measurements in order to investigate the dependence of the flexibility of DNA on ionic strength and counterion valence. In the second phase of this project, other factors which are thought to influence the conformation and flexibility of DNA will be studied. These include: intercalating agents, single-strand nicking, and base sequence. The temperature-dependence of flexibility will also be studied in an effort to obtain occurate values for the associated thermodynamic quantities. The third phase of this research will involve a study of the electric-field-induced melting of DNA, since this approach holds promise as a means of probing the arrangement of counterions around the DNA molecule.